Reflections caused by connector interfaces “blind” OTDRs for a short period of time. The period of time it takes for an OTDR to recover from this “blindness” is commonly referred to as a “dead-zone”. During this dead-zone period, OTDRs are unable to distinguish one anomaly (e.g. breaks, shatters, bends, or even another connector) from another.
OTDR launch cables, or dead-zone boxes, placed at near-end patch panels allow the OTDRs detector port sufficient time to recover from its blindness. Dead zone boxes also enable the OTDR to measure the relative loss through the near-end and far-end patch panels (see diagram below).
Without a sufficiently long launch cable, the reflection from the near-end patch panel will be undetectable because it is within the dead-zone caused by the OTDR port. Likewise, at the far-end, lack of a dead-zone box will prevent the OTDR from measuring the relative loss through the far-end patch panel.
In most cases,
launch cables that are slightly longer than one kilometer are sufficient to account for
dead-zones on nearly all fiber links. However, longer launch cables may be needed for long-haul fiber optic links.
